From 6960c519f143ad68a690fbb3e0ef94b685fdcbbc Mon Sep 17 00:00:00 2001
From: rymnc <43716372+rymnc@users.noreply.github.com>
Date: Sun, 29 Dec 2024 22:48:45 +0530
Subject: [PATCH] test: conditionally use simd variant

---
 fuel-vm/src/interpreter/memory.rs | 98 +++++++++++++++++++++----------
 1 file changed, 68 insertions(+), 30 deletions(-)

diff --git a/fuel-vm/src/interpreter/memory.rs b/fuel-vm/src/interpreter/memory.rs
index 9472547dc..d997313d8 100644
--- a/fuel-vm/src/interpreter/memory.rs
+++ b/fuel-vm/src/interpreter/memory.rs
@@ -1,7 +1,7 @@
 #![cfg(feature = "alloc")]
 
 #[cfg(feature = "experimental")]
-use core::simd::u8x16;
+use core::simd::{u8x16, u8x32, u8x64};
 
 use super::{
     internal::inc_pc,
@@ -1040,37 +1040,75 @@ fn slice_eq(a: &[u8], b: &[u8]) -> bool {
             return false;
         }
 
-        // because we're using simd, we can parallelize by a factor of 4
-        let mut res_a = false;
-        let mut res_b = false;
-        let mut res_c = false;
-        let mut res_d = false;
-
-        let chunks_a = a.chunks_exact(16);
-        let chunks_b = b.chunks_exact(16);
-        let remainder_a = chunks_a.remainder();
-        let remainder_b = chunks_b.remainder();
-
-        for i in (0..chunks_a.len()).step_by(4) {
-            let simd_a = u8x16::from_slice(&a[i..i + 16]);
-            let simd_b = u8x16::from_slice(&b[i..i + 16]);
-            res_a |= simd_a != simd_b;
-
-            let simd_a = u8x16::from_slice(&a[i + 16..i + 32]);
-            let simd_b = u8x16::from_slice(&b[i + 16..i + 32]);
-            res_b |= simd_a != simd_b;
-
-            let simd_a = u8x16::from_slice(&a[i + 32..i + 48]);
-            let simd_b = u8x16::from_slice(&b[i + 32..i + 48]);
-            res_c |= simd_a != simd_b;
-
-            let simd_a = u8x16::from_slice(&a[i + 48..i + 64]);
-            let simd_b = u8x16::from_slice(&b[i + 48..i + 64]);
-            res_d |= simd_a != simd_b;
+        enum SimdVariant {
+            U8x16,
+            U8x32,
+            U8x64,
         }
 
-        // check the remainder
-        remainder_a == remainder_b && res_a && res_b && res_c && res_d
+        impl SimdVariant {
+            fn chunk_size(&self) -> usize{
+                match &self {
+                    SimdVariant::U8x16 => 16,
+                    SimdVariant::U8x32 => 32,
+                    SimdVariant::U8x64 => 64,
+                }
+            }
+        }
+
+        // we check if the number of bytes is divisible by 16, 32, 64.
+        // we choose the largest number that has the smallest remainder
+
+        let remainder_16 = a.len() % 16;
+        let remainder_32 = a.len() % 32;
+        let remainder_64 = a.len() % 64;
+
+        // based on the smallest remainder, we then use the appropriate simd instruction
+        // to compare the slices
+        let variant = if remainder_16 < remainder_32 {
+            if remainder_16 < remainder_64 {
+                SimdVariant::U8x16
+            } else {
+                SimdVariant::U8x64
+            }
+        } else {
+            if remainder_32 < remainder_64 {
+                SimdVariant::U8x32
+            } else {
+                SimdVariant::U8x64
+            }
+        };
+
+        let chunks = a.chunks_exact(variant.chunk_size());
+        let remainder = chunks.remainder();
+
+        for (chunk_a, chunk_b) in chunks.zip(b.chunks_exact(variant.chunk_size())) {
+            match variant {
+                SimdVariant::U8x16 => {
+                    let simd_a = u8x16::from_slice(chunk_a);
+                    let simd_b = u8x16::from_slice(chunk_b);
+                    if simd_a != simd_b {
+                        return false;
+                    }
+                }
+                SimdVariant::U8x32 => {
+                    let simd_a = u8x32::from_slice(chunk_a);
+                    let simd_b = u8x32::from_slice(chunk_b);
+                    if simd_a != simd_b {
+                        return false;
+                    }
+                }
+                SimdVariant::U8x64 => {
+                    let simd_a = u8x64::from_slice(chunk_a);
+                    let simd_b = u8x64::from_slice(chunk_b);
+                    if simd_a != simd_b {
+                        return false;
+                    }
+                }
+            }
+        }
+
+        remainder == &b[a.len() - remainder.len()..]
     }
     #[cfg(not(feature = "experimental"))]
     {